Optical fiber cables typically include a plurality of thin glass or plastic optical strands or “fibers,” each of which is configured as a medium through which high-frequency light-wave signals may be passed. For protection and organization, the plurality of fibers may be divided into groups, with each group being placed within its own protective tube or channel called a “buffer” tube.
There are various configurations of optical fiber cables for transmitting information, including: central loose tube cables, which generally comprise a central buffer tube containing one or more optical fibers; stranded loose tube cables, which generally comprise a plurality of buffer tubes stranded in a helical or reverse oscillation (“S-Z”) lay around a central strength member; and cables with slotted cores, which generally comprise a solid central core member having a plurality of grooves formed within the outer edge of the core member and into which a plurality of fibers may be placed.
A stranded loose tube cable typically includes a plurality of optical fibers that may be divided into groups, with each group being placed within a protective plastic or polymeric buffer tube. The buffer tubes are stranded around a central strength member, which increases the tensile strength of the cable and limits tensile and/or compressive stress that may be imposed upon the individual fibers during installation and use. The buffer tubes and central strength member may be housed in a protective polymeric jacket or sheath, which limits the exposure of the optical fibers to the surrounding environment. The buffer tubes and protective polymeric jacket may be filled with water-blocking substances and/or materials for blocking the ingress/propagation of water both inside the buffer tubes and in the interstices between the buffer tubes and the protective jacket. See, for example, U.S. Pat. No. 5,229,851, which is incorporated herein by reference in its entirety.
Typically, stranded loose tube cables have a plurality of buffer tubes, each buffer tube having a plurality optical fibers disposed therein. In an effort to maximize the structural integrity of the cable, the buffer tubes, central strength member, and diameter of the protective jacket may each be designed to limit or minimize movement of the buffer tubes within the protective jacket. One exemplary design uses six buffer tubes, each sized appropriately to fit precisely between the central strength member and the inner diameter of the protective jacket, thereby minimizing movement of the position of each buffer tube within the cable.
Cable manufacturers often customize certain design criteria of the optical fiber cable. For example, one customer may require a 72-fiber optical fiber cable (six buffer tubes, each containing 12 fibers), while another customer may require a 60-fiber cable (five buffer tubes, each containing 12 fibers). In order to flexibly accommodate customer requirements and maintain structural integrity of the cable without requiring a complete redesign of the optical fiber cable, cable manufacturers may replace any unused buffer tubes with dummy rods. Dummy rods are elongated elements typically constructed of polymeric material that have the same or similar features (e.g., diameter, water blocking capabilities, etc.) as the buffer tubes. Importantly, dummy rods are substantially less expensive to manufacture than buffer tubes containing optical fibers and waterproofing materials. As such, where customer specifications require fewer buffer tubes than a particular type of optical fiber cable was designed to incorporate, dummy rods having the same dimensions as buffer tubes but lacking any optical fibers may be substituted for buffer tubes and perform as “simulated buffer tubes.”
Dummy rods help to maintain the structural integrity of the cable when fewer buffer tubes are desired for a particular cable design, thereby mitigating costs associated with manufacturing a customized cable. Furthermore, the use of dummy rods allows cable manufacturing lines to make cables of varying fiber counts without substantial modification to the equipment setup, thereby avoiding otherwise costly variations to the cable manufacturing and processing equipment. For example, cables having 12, 18, 24, or 36 fiber counts can be manufactured having the same basic cable structure and diameter and using the same cable processing equipment, despite having differing fiber counts and/or requiring different numbers of buffer tubes.
Conventionally dummy rods are designed with substantially the same diameter as the buffer tubes so that the stranding of the buffer tubes (and dummy rod(s)) is uniform without creating space for the buffer tubes to move. The dummy rod is generally made of similar material(s) as the buffer tubes to insure that thermal and mechanical performance is the same as the buffer tubes.
For example, U.S. Pat. No. 4,550,976 (“the '976 patent”), which is incorporated herein by reference in its entirety, teaches a “dummy tube” having essentially the same outside diameter as the buffer tube and comprising a core means “circumscribed” by a coating of foamed plastic. The core may be a monofilament (or a plurality of monofilament strands twisted around one another) or a metallic material, such as steel. The coating of foamed plastic is then “circumscribed” about the monofilament or metallic core.
The '976 patent notes that conventional dummy tubes are typically constructed as a solid rod of the same material used to make the buffer tubes. Furthermore, the '976 patent suggests that materials used to make buffer tubes tend to be expensive and that certain other, cheaper materials may be substituted without having an adverse effect on dummy tube performance. Accordingly, the '976 patent suggests that a less expensive dummy tube may be constructed using a monofilament or steel core surrounded by a less expensive foamed plastic than is used for the buffer tubes.
Another dummy rod design is described in U.S. Pat. No. 6,066,397 (“the '397 patent”), which is incorporated herein by reference in its entirety. The '397 patent describes a dummy rod (therein named as “filler rod”) that is constructed of a polypropylene homopolymer, a polypropylene-polyethylene copolymer resin, or a polypropylene-polyethylene copolymer having a nucleating agent disbursed therein. The resin material is foamed during extrusion so as to create a plurality of void spaces, which aids in reducing density relative to unfoamed material.
The foamed dummy rod design of the '397 patent may realize a reduction in density when compared to conventional “solid” dummy rods. Such density reduction may use less material during manufacture and, therefore, may reduce costs associated with manufacturing the dummy rods. However, the foamed dummy rod of the '397 patent does not appear to provide a significant reduction in cross-sectional area relative to conventional “solid” dummy rods. By not significantly reducing the cross-sectional area of the dummy rod, the overall reduction in material cost realized by foamed dummy rods when compared with the conventional dummy rod designs may be limited.
U.S. Pat. No. 6,922,512 (“the '512 patent”) describes a fiber optic cable having a plurality of buffer tubes and filler rods disposed between the inner surface of the outer jacket of the fiber optic cable and the buffer tubes containing a plurality of optical fibers. The cross-sectional shape of the filler rods of the '512 patent is designed to match the cross-sectional shape of the interstitial gaps found in the cable between the buffer tubes and the cable outer jacket, of the cable. The filler rods have a core and are surrounded by a super-absorbent water-swellable coating, which helps the filler rods limit water penetration between gaps formed at the intersection of the buffer tubes and the outer jacket of the fiber optic cable. To promote increased adherence of the water-swellable coating to the core, the core of the filler rods disclosed in the '512 patent is non-round, which increases the surface area of the core that is available for contact with and adherence to the water-swellable coating.
Although the filler rods of the '512 patent have non-circular cores, nowhere does the '512 patent suggest to replace any buffer tube with a filler rod as described therein. The cross-sectional shape of such filler rods is taught to have been designed for performing a different function.
Applicant has identified that known designs for stranded loose-tube cables with dummy rods, such as those described above, have several manufacturing and operational deficiencies. In particular, conventional dummy rods, even those formed by simple extrusion, still tend to require an excessive amount of material, which increases cable costs. Furthermore, Applicant has also observed that filler rods designed for a function other than replacing buffer tubes and having a non-circular cross-sectional configuration lack cross-sectional dimensions that make them suitable replacements for buffer tubes in stranded loose-tube cables, particularly because the shapes of said filler rods do not encompass a cross-sectional shape and area that is substantially similar to that of the buffer tube.